The present invention relates to a method for making an anode foil for an aluminum electrolytic capacitor used in various electronic appliances.
In the conventional manufacturing- process of an anode foil for an aluminum electrolytic capacitor, to feed current in the anodizing step, an aluminum foil of which surface is roughened by etching is brought into contact with a current feeding roller to feed current. In this method, however, aluminum powder may deposit on the surface of the current feeding roller, or spark discharge may occur between the current feeding roller and the aluminum foil, when an amount of feeding current is increased.
To solve these problems, a method of feeding current in aqueous solution is proposed. For example, Japanese Patent Publication No. 03-81292 (1991) discloses an art of anodization by conveying an aluminum foil in the sequence of a step of forming false boehmite by immersion treatment in a high-temperature deionized water, a first anodizing step of applying a anodizing voltage in a range of 10 to 100 V, a step of feeding current to the aluminum foil in a current feeding cell, and a second anodizing step. However, when current is fed in a current feeding cell in which a current feeding electrode is disposed, the current may be locally concentrated between the electrode and aluminum foil, and, in this portion, the aluminum foil may be distorted or the end may be torn due to hydrogen embrittlement. As practical means for preventing this phenomenon, electrochemical forming apparatuses disposing electrical insulating shielding plates are proposed in Japanese Utility Model Publication Nos. 62-20997 (1987) and 62-18029 (1987).
However, in such conventional anodizing methods of using the current feeding cell disposing the current feeding electrode, when the amount of feeding current from the current feeding cell increases, even if the voltage range of the first anodizing step is defined or the insulating shielding plate is disposed, distortion of aluminum foil or tearing of end portion due to current concentration may be caused. The higher the anodizing voltage in the first anodizing step, the higher is the value of resistance of the anodized film, and it causes to elevate the bath voltage in the current feeding cell or promote the phenomenon of hydrogen embrittlement due to current concentration, or the power consumption in the current feeding cell may be increased.
A first aspect of the invention relates to a method for making an anode foil which comprises a first anodizing step of forming an anodized film on the surface of an aluminum foil, an immersion treatment step of immersing the aluminum foil in an aqueous solution containing an alkaline metal compound, a current feeding step of feeding an electric current to the aluminum foil in a current feeding solution in a current feeding cell disposing a current feeding electrode connected to the anode of a direct current power source, and a second anodizing step of forming an anodized film on the surface of the aluminum foil by applying a higher voltage than the voltage in the first anodizing step.
According to this anodizing manufacturing method while continuously conveying the aluminum foil roughened on the surface, the anodized film formed on the surface of the aluminum foil in the first anodizing step is immersed in the aqueous solution containing alkaline metal ions while having numerous film defects, and the aluminum foil having alkaline metal ions depositing on its surface is immersed in the current feeding solution in the current feeding cell. In the current feeding cell, the aluminum foil acts as the cathode, and therefore the alkaline metal ions charged positively gather on the surface of the aluminum foil without dispersing in the current feeding solution in the current feeding cell. Therefore, it is alkaline locally, and the anodized film is dissolved. In particular, in the chemical film defect areas, defects are exposed, and the current feeding solution penetrates into the exposed defects, and the film resistance becomes smaller. As a result, if the anodizing voltage is high, the resistance of the anodized film can be lowered, and the current density in the current feeding cell is uniform, and the bath voltage in the current feeding cell drops. At the same time, the generated hydrogen is released to the current feeding solution side through the exposed defects, and occurrence of hydrogen embrittlement is prevented, and the amount of feeding current in the current feeding cell is increased. Therefore, the limitation of the first anodizing voltage is lifted, and the installation of insulating shielding plate is not necessary, and also the power consumption is saved and the productivity is enhanced.
In order to obtain better effects, the alkaline metal compound to be added in the aqueous solution in the immersion treatment step is preferred to be any compound of Li, Na, K, Rb, or Cs. The concentration of the alkaline metal compound to be added in the aqueous solution in the immersion treatment step is preferred to be 0.1 ppm or more.
A second aspect of the invention relates to a method for making an anode foil, for forming an anodized film while continuously conveying an aluminum foil of which surface is roughened, which comprises a first anodizing step of forming an anodized film on the surface of an aluminum foil, a current feeding step of feeding an electric current to the aluminum foil in a current feeding solution containing an alkaline metal compound in a current feeding cell disposing a current feeding electrode connected to the anode of a direct-current power source, and a second anodizing step of forming an anodized film on the surface of the aluminum foil by applying a higher voltage than the voltage in the first electrochemical forming step.
According to this manufacturing method, the anodized film formed on the surface of the aluminum foil in the first anodizing step is immersed in the current feeding solution in the current feeding cell while having numerous film defects, and the aluminum foil acts as the cathode in the current feeding cell. Therefore, the alkaline metal ions charged positively gather on the surface of the aluminum foil, and it is alkaline locally, and the anodized film is dissolved. In particular, in the anodized film defect areas, defects are exposed, and the current feeding solution penetrates into the exposed defects, and the film resistance becomes smaller. As a result, if the first anodizing voltage is high, the resistance of the anodized film can be lowered, and the current density in the current feeding cell is uniform, and the bath voltage in the current feeding cell drops. At the same time, the generated hydrogen is released to the current feeding solution side through the exposed defects, and occurrence of hydrogen embrittlement is prevented, and the amount of feeding current in the current feeding cell is increased. Therefore, the limitation of the first anodizing voltage is lifted, and the installation of insulating shielding plate is not necessary, and also the power consumption is saved and the productivity is enhanced.
In order to obtain better effects, the alkaline metal compound to be added in the current feeding solution in the current feeding step is preferred to be any compound of Li, Na, K, Rb, or Cs. The concentration of the alkaline metal compound to be added in the aqueous solution in the current feeding step is preferred to be 0.1 ppm or more.